The undisputed role of the cerebral cortex in perception, action and higher cognitive processes has led to an intense effort to understand the functional properties of this tissue. A primary goal of this effort has been to understand the mechanisms whereby individual cortical neurons obtain their unique repertoire of response properties and how these properties contribute to overall cortical function. The proposed research is aimed at understanding the elaboration of response properties seen in identified neurons in the intact, thalamo-cortical somatosensory system. Two sets of experiments will be conducted, all in fully awake rabbits. (1) The first set is aimed at understanding the transformations performed upon inputs to the cortex by the intracortical circuitry and how these transformations lead to parallel and distinct efferent outflows. The activity of thalamo-cortical projection neurons, corticocortical and corticifugal efferent neurons and putative interneurons will be simultaneously recorded and subjected to cross-correlational analysis. Previous work has resulted in a model of S-1 cortical circuitry that has generated explicit predictions of the excitatory and inhibitory synaptic interactions to be expected among these identified elements. These experiments will test this model and result in its further elaboration. (2) The second set of experiments examines the nature of a large population of neurons throughout sensory cortex that have no demonstrable (supra-threshold) receptive fields. It has recently been shown, however, that most such neurons do have sub-threshold receptive fields. This proposal will use a new technique for analyzing sub-threshold synaptic events to identify which classes of cortical neurons demonstrate sub- threshold receptive fields and to explore the spatial and temporal characteristics of such fields. In addition, the pharmacological conditions under which sub-threshold receptive fields become supra- threshold will be determined. The data will offer a unique view of the physiology of individual cortical neurons studied under natural conditions, and will provide insight into interactive networks of cortical neurons and their varied functional properties.